The present invention relates to a method for installing piles in offshore locations. More particularly, the invention relates to installing piles using what is commonly referred to as a `drill and grout` method as contrasted to mechanically driving the pile into the ocean floor. In the drill and grout method, a borehole is drilled at the desired location and the pile is installed and cemented in place.
In the past, the reliability and load-carrying capacity of a drilled and grouted pile in an offshore location has been questioned. The concern arises from the inability to obtain reliable placement of the grouting cement around the pile and the inability to ensure that the grout obtained a good contact with both the surface of the pile and the surface of the borehole. It is obvious that if good contact is not achieved between the surface of the pile and the surface of the borehole, the load-carrying capacity of the pile is seriously reduced. The reliability of the contact of the grouting cement with the wall of the borehole is seriously affected by the mudcake that is deposited on the borehole wall from the drilling fluid used in drilling the borehole. While various methods have been proposed in the prior art for dealing with this problem, none are entirely reliable. Examples of such prior art are U.S. Pat. Nos. 2,649,160, 3,499,491 and 3,557,876. All of these patents describe methods for cementing casing in oil wells and describe various means for improving the contact between the cement and the wall of the borehole. In addition, the patents describe various ways for removing or neutralizing the effect of the mudcake deposited on the borehole wall from the drilling fluid used in drilling the borehole.
In all of the above systems Portland cement was used as the grouting material. The high viscosity and weight of Portland cement creates several problems when using it to grout piles in offshore locations. One problem caused by the high viscosity of Portland cement is the difficulty involved in attempting to place the cement in the annulus between the pile and the borehole wall. Normally, the pile will not be centered in the borehole and Portland cement will tend to flow towards the side of the pile that is the greatest distance from the borehole wall and force the opposite side of the pile into contact with the wall. This reduces the grouting on this side of the pile and the load-carrying capacity of the pile is reduced.
A low viscosity cement useful as a grouting composition for well casing is disclosed in U.S. Pat. No. 4,761,183. The cement is described as a mixture of blast furnace slag and water in which the ratio of water to blast furnace slag is between 3:1 and 12:1. The high water content is used to produce a low viscosity, flowable grouting material. In contrast, as the water content of a Portland cement slurry is increased the strength of the grout is reduced.
If the load-carrying ability of a drilled and grouted pile could accurately be predicted it would allow the use of conventional drilling rigs for installing piles in offshore locations. This would eliminate the expense of transporting special pile driving equipment to the location to install the piles. For example, if it is desired to use a conventional drilling rig to install piles using mechanical pile driving means, it is normally necessary to remove considerable equipment such as blowout preventors and drilling risers from the rig so that the pile driving equipment can be installed on the rig. As the exploration in offshore waters moves into deeper water and more exposed locations, this requires that the rig be moved to protected waters to effect the removal of the drilling equipment and the installation of the pile driving equipment. Any move may require removal of the drilling riser from the rig and this normally requires barges for storing the drilling riser. All of the above greatly increases the expense of installing piles in offshore locations whether special equipment is used or the drilling rig is used.